Newton’s First LawLevel 1 Physics

CHAPTER 4ESSENTIAL QUESTIONS

What are Newton’s Laws of Motion?

How do forces cause motion?

How can the Law of Universal Gravitation be applied?

How can one weigh the earth?

What effect does friction have on the motion of an object?

OBJECTIVES FORNEWTON’S LAWS

Force FactsBasic definition: a push or a pull

Can we do a better job? Probably.

An interaction between bodies.

Forces can be classified by two types;1. Contact Force

i.e. Normal, Tension, Friction

2. Field Force (at-a-distance)i.e. Gravitational, Magnetic, electric

Newton’s First LawThe Law of Inertia

The natural tendency for all objects is motion

Newton’s First Law (N.F.L.) An object remains at rest or in a state of motion at constant speedalong a straight line unless compelled to change that state by a net force (F).

Forces are balanced

Object at rest vo = 0 m/s

Stays at rest

a = 0 m/s/s

Object in motion v0 0 m/s/s

Object in motionSame speed/direction

a = 0 m/s/s

InertiaInertia – Tendency to resist motion

The more massive an object is, the more inertia it will haveIt is harder to get boulder in moving (also harder to stopthe boulder from moving) than it is to bet a pebble moving

Inertia is a quantity that is dependent upon the mass of the object

Free Body Diagrams(FBD)

A pictorial representation of all of the forces acting on an object

N

T

mg

f

T

mg

Normal force (N): Surfaceforce that is always drawnperpendicular to thesurface

Weight (mg): Always drawnStraight down from center

Tension (T): Force inropes/chains drawnaway from object

Friction (f): Drawn opposing motion

FBD for slopes

mg

Nf

A 10-kg box is being pulled across the table to the right at a constant speed with a force of 50N.

a) Calculate the Force of Friction

a) Calculate the Force Normal

mg

N

Ff €

F = f = 50N

€

mg = N = (10)(9.8) = 98N

Example

Suppose the same box is now pulled at an angle of 30 degrees above the horizontal.

a) Calculate the Force of Friction

a) Calculate the Force Normal

mg

N F

f 30

€

Fx = F cosθ = 50cos30 = 43.3N

f = Fx = 43.3N

Fx

Fy

€

N ≠ mg!

N + Fy = mg

N = mg− Fy →(10)(9.8) −50sin30

N = 73N

Example

A Final Note